I spent some pleasant time as a docent in an Automobile Museum. There I learned why the early cars -- 1900s to 1920s -- were almost all open bodied cars. It wasn't that the weather was nicer then. And it wasn't that people were hardier then. It was the grotesque hazard of plate glass.
"Safety glass" was invented in the late 1920s, and pretty much revolutionized the automobile industry. (1928 Model A Ford.)
There are two kinds.
Tempered glass, which shatters relatively harmlessly into fourtyeleven gazillion "pebbles". It is used in side- and rear- windows. It can not be cut, and is not sold over the counter for automotive use.
Laminated glass. It is used in windshields, because it does NOT shatter in fourtyeleven gazillion pieces, but stays somewhat together so you can see to bring the car to a safe stop. Flat laminated glass is sold over the counter and can be cut to any size.
Laminated glass is what you want to put in your bus. See your local automobile glass shop for custom cuts. Or, of course, use automotive, bus and RV windows.
No residential plate glass, PLEASE!